maybe not in all forums, but like under the permission when you edit a forum could you add an option for allowing guests to post an attchment?

This way it can be restricted to certain forums. Like for example:

I am a game maker, I have a site for the project team, but we want to have a public forum where guest users can donate artwork and work to help move the games along, and while we would not want the guests to be able to post on every forum if you could choose that right individually.